An important function of the vertebrate kidney is the excretion of potentially toxic chemicals, such as waste products of cellular metabolism, xenobiotics and xenobiotic metabolites. This occurs primarily in the proximal tubule, where renal transport systems remove organic anions and organic cations from peritubular capillaries and transport them across the tubular epithelium into the lumen. Much work has focussed on the mechanisms by which organic anions and organic cations are transported across the surface membranes of the proximal tubular cells, but little attention was paid to the routes by which these compounds cross the cell interior, the assumption being that simple diffusion across the cytoplasm was the only process necessary. We are using comparative renal models (proximal tubules from lower vertebrates and invertebrates and mammalian renal cells in culture) in combination with fluorescence microscopy (conventional and confocal), video imaging, intracellular microinjection and isolated membrane vesicle techniques to test that assumption. We have found that, after transport into renal cells, organic anions and cations are distributed over two compartments: one is diffuse and cytoplasmic, and the second punctate and vesicular. Sequestration in intracellular vesicles is not a result of endocytosis, but rather of active uptake from the cytoplasm. Such uptake is concentrative, specific and energy-dependent. Thus, organic anions and cations are not uniformly distributed within cells, rather, for each, a substantial fraction is sequestered in vesicles. For organic anions, initial results indicate that these vesicles move through the cytoplasm in the secretory direction on microtubules and that this movement contributes substantially to the rate of net secretion. Future studies will focus on: 1) further characterizing the vesicular transport processes involved, 2) defining the role that vesicular transport plays in overall renal xenobiotic secretion, and 3) searching for analogous mechanisms in other specialized epithelia that accumulate and transport organic anions and cations, e.g., liver and choroid plexus.